1. Field of the Invention
The present invention relates to a large bulk material of a rare-earth-element oxide superconductor and a method for manufacturing the same.
2. Description of the Related Art
As a conventional method of manufacturing a superconductive bulk material of REBa2Cu3Ox system with which the present invention is concerned, there has been a melting method as represented by the Quench and Melt Growth method (Patent Registration No. 1869884 and Patent Registration No. 2556401). This method is as follows: first, materials are heated up to a temperature of the region where RE2BaCuO5 phase or RE4Ba2Cu2O10 phase and a liquid phase including Baxe2x80x94Cuxe2x80x94O compounds as a main component coexist, then, the materials are cooled to a level immediately above a peritectic temperature at which the REBa2Cu3Ox phase generates, the material are further cooled slowly to generate crystals, and nucleus generation and crystal orientation in the material are controlled by the cooling to obtain a large bulk. Based on this manufacturing method, it is possible to obtain a relatively large superconductor having a high transition current density (a current density per unit cross section as one of superconductive characteristics).
Further, as a method for manufacturing a much larger superconductive bulk material, there has been known a method for using one seed crystal and sequentially combining materials having different starting-temperatures of crystal-growth together, and controlling a nucleus formation, a crystal orientation and a crystal growth direction (conventional method 1) (Japanese Patent Application Laid-open Publication No. 5-170598). According to this method, it is possible to manufacture a material having a diameter exceeding 100 mm. This method, however, has a problem in that it takes a long time for a crystal growth, one month as a maximum, and has low productivity.
In the mean time, there has also been known a method for manufacturing a large material by growing a crystal from various seed crystals (conventional method 2). According to this method, a region for growing a crystal from each seed crystal becomes small, and therefore, this method has an advantage that it is possible to achieve a crystal growth in a relatively short time. Further, according to this method, a transition current density is high within a region (A) in which a crystal has grown from each seed crystal. However, as shown in FIG. 1 and FIG. 2, mainly a Baxe2x80x94Cuxe2x80x94O compound and a Cuxe2x80x94O compound, or a segregated RE2BaCuO5 phase or RE4Ba2Cu2O10 phase are precipitated as excluded phases between the regions (A). As a result, the transition current density between these regions is lowered extremely. Therefore, this method has had a problem in that the characteristics of a superconductive bulk material manufactured by this method are extremely inferior to those of a material manufactured by the conventional method 1, as a total superconductive bulk unit. As a method of improving this situation, it has been reported that the distributing the seed crystals with smaller intervals (about 5 mm) between them will make it difficult to generate impurity phases. According to this improvement method, the whole surface for a crystal growth must be embedded with the seed crystals. Thus, this method has not been practical from the viewpoint of productivity. Therefore, there has been demanded a superconductive bulk unit and a method for manufacturing this superconductive bulk unit that makes it difficult to generate, or that does not generate, an excluded phase between the regions (A).
It is, therefore, an object of the present invention to provide a large superconductive bulk material and a method for manufacturing this large superconductive bulk material capable of reducing or eliminating excluded phases such as a Baxe2x80x94Cuxe2x80x94O compound and a Cuxe2x80x94O compound, or a segregated RE2BaCuO5 phase or RE4Ba2Cu2O10 phase that are precipitated between one region of crystal growth from one seed crystal and the other region of crystal growth from the other seed crystal, and capable of making a larger current to flow between the regions of crystal growth from the seed crystals.
The present invention solves the above problems. An oxide superconductor is structured by oxide superconductors of 123 phases of two or more kinds of different peritectic temperatures (Tp). Seed crystals are disposed at the oxide superconductor side having a highest Tp, and crystals are grown. Then, it has been found that excluded phases of a Baxe2x80x94Cuxe2x80x94O compound and a Cuxe2x80x94O compound, or a segregated RE2BacuO5 phase or RE4Ba2Cu2O10 phase are generated in the layers of superconductors having a high Tp, and that these excluded phases do not propagate easily in the superconductor layers having a low Tp. Based on this finding, the present invention provides a superconductive bulk unit that has reduced or eliminated xe2x80x9cexcluded phasesxe2x80x9d that interrupt a current flow in the superconductive bulk unit, at the portion of the superconductive layers having lower Tp""s. As a result, the superconductive bulk unit can make a larger current to flow based on the use of the superconductor layers having a low Tp.
According to a first gist of the present invention, there is provided a large superconductor intermediate of REBa2Cu3Ox system (where RE is one kind or a combination of rare earth elements including Y), characterized by a structure that oxide superconductors having non-superconductive phases finely dispersed in REBa2Cu3Ox phases (123 phases) of different peritectic temperatures (Tp) are laminated three-dimensionally in the order of Tp""s, seed crystals mounted on the oxide superconductor layer having a highest Tp, and excluded phases included in at least the oxide superconductor having the high Tp.
In the present invention, the REBa2Cu3Ox system (where RE is one kind or a combination of rare earth elements including Y) means that a base phase is not only REBa2Cu3Ox, but also is (RE1xe2x88x92zBaz)1+yBa2xe2x88x92yCu3Ox (xe2x88x920.5xe2x89xa6yxe2x89xa61.0, 0xe2x89xa6zxe2x89xa60.5). Further, this also includes a material of which a part of or the whole Ba has been substituted by Sr. Similarly, the REBa2Cu3Ox (123) phases as the base phases include not only REBa2Cu3Ox, but also (RE1xe2x88x92zBaz)1+yBa2xe2x88x92yCu3Ox (xe2x88x920.5xe2x89xa6yxe2x89xa61.0, 0xe2x89xa6zxe2x89xa60.5). Further, they also include a material of which a part of or the whole Ba has been substituted by Sr.
The non-superconductive layer in the present invention means the RE2BaCuO5 phase or the RE4Ba2Cu2O10 phase finely distributed in the REBa2Cu3Ox phases (123 phases) as the base phases. Further, the REBa2Cu3Ox phases (123 phases) of different peritectic temperatures (Tp) means a oxide superconductor structured by composition powders of different Tp""s having different peritectic temperatures (Tp) of the 123 phases by changing the RE compositions or changing the addition volumes of the added elements like Ag. Further, the structure obtained by a three-dimensional lamination means a lamination in layers, or a concentric structure, or a combination of these.
According to a second gist of the present invention, there is provided a large superconductor intermediate of the above first gist, wherein a deviation of a crystal orientation between the nearest seed crystals is within 30 degrees in the mounting of seed crystals. The nearest seed crystals in the present invention means the seed crystals that are located at the nearest positions among various disposed seed crystals. Further, the deviation in the crystal orientation means a minimum rotational angle that is necessary for matching a crystal orientation of a certain seed crystal with a crystal orientation of the nearest seed crystal by rotating these seed crystals in space.
According to a third gist of the present invention, there is provided a large superconductor intermediate of the above first or second gist, wherein the laminated structure includes at least one oxide superconductor that includes at least one of Rh of 0.001 to 2.0 mass %, Pt of 0.05 to 5.0 mass %, and Ce of 0.05 to 10.0 mass %.
According to a fourth gist of the present invention, there is provided a large superconductor prepared by carrying out a removal of an oxide superconductor that includes seed crystals or excluded phases and/or an oxygen enrichment heat treatment, to a large superconductor intermediate in any one of the above first to third gist.
According to a fifth gist of the present invention, there is provided a method for manufacturing a large superconductor intermediate, comprising the steps of heating raw material molds including RE, Ba and Cu that constitute a superconductor of the REBa2Cu3Ox system (where RE is one kind or a combination of rare earth elements including Y), heating the molds to a level equal to or above a peritectic temperature (Tp) of REBa2Cu3Ox phases (123 phases) of the molds as a maximum temperature, and cooling the molds to manufacture an oxide superconductor intermediate having non-superconductive phases finely dispersed in the 123 phases, characterized by laminating three-dimensionally the raw material molds of 123 phases having different Tp""s in the order of Tp""s to form a raw material mold laminate, mounting seed crystals on the raw material mold having a highest Tp, and generating a crystal growth of the raw material mold laminate.
According to a sixth gist of the present invention, there is provided a method for manufacturing a large superconductor intermediate of the above fifth gist, wherein a deviation of a crystal orientation between the nearest seed crystals is within 30 degrees in the mounting of seed crystals.
According to a seventh gist of the present invention, there is provided a method for manufacturing a large superconductor intermediate of the above fifth or sixth gist, wherein the raw material mold laminate includes at least one raw material mold that includes at least one of Rh of 0.001 to 2.0 mass %, Pt of 0.05 to 5.0 mass %, and Ce of 0.05 to 10.0 mass %.
According to an eighth gist of the present invention, there is provided a method for manufacturing a large superconductor of the REBa2Cu3Ox system (where RE is one kind or a combination of rare earth elements including Y), comprising the steps of carrying out a removal of an oxide superconductor that includes seed crystals or excluded phases from the large superconductor intermediate manufactured by the method in any of the above fifth to seventh gist, and/or carrying out an oxygen enrichment heat treatment to this intermediate.